Light-Confining Nanoporous Anodic Alumina Microcavities by

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Light-Confining Nanoporous Anodic Alumina Microcavities by Apodized Stepwise Pulse Anodization Cheryl Suwen Law, Siew Yee Lim, Raeanne Macalincag, Andrew D. Abell, and Abel Santos ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00494 • Publication Date (Web): 28 Aug 2018 Downloaded from http://pubs.acs.org on August 31, 2018

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ACS Applied Nano Materials

Light-Confining Nanoporous Anodic Alumina Microcavities by Apodized Stepwise Pulse Anodization Cheryl Suwen Law1,2,3, Siew Yee Lim1,2,3, Raeanne M. Macalincag1, Andrew D. Abell2,3,4*, and Abel Santos1,2,3* 1

School of Chemical Engineering, The University of Adelaide, SA 5005 Adelaide, Australia. Institute for Photonics and Advanced Sensing (IPAS), The University of Adelaide, SA 5005 Adelaide, Australia. 3 ARC Centre of Excellence for Nanoscale BioPhotonics (CNBP), The University of Adelaide, SA 5005 Adelaide, Australia. 4 Department of Chemistry, The University of Adelaide, SA 5005 Adelaide, Australia. 2

*E-mails: [email protected] ; [email protected] KEYWORDS: Nanoporous Anodic Alumina, Optical Microcavity, Light Confinement, Apodized Anodization, Quality Factor. ABSTRACT: This study presents an innovative approach to fabricate nanoporous anodic alumina optical microcavities (NAA-µCVs) with enhanced quality factor and versatile optical properties. An apodization strategy using a logarithmic negative function is applied to a stepwise pulse anodization process in order to engineer the effective medium of NAA so it confines light efficiently. The architecture of these light-trapping photonic crystals is composed of two highly reflecting mirrors with asymmetrically apodized effective medium. Various anodization parameters such as anodization time, anodization period, current density offset, and pore widening time are systematically modified to assess their effect on the optical properties of NAA-µCVs in terms of quality factor and position of resonance band. We demonstrate that this fabrication approach enables the generation of NAA-µCVs with high quality factor (~113) and well-resolved and tunable resonance bands across the spectral regions, from UV to NIR, through the manipulation of the anodization parameters. These results represent a comprehensive rationale for the development of high quality NAA-µCVs with enhanced light-confining capabilities, providing new opportunities for further fundamental and applied research across a broad range of fields and disciplines such as photonics and optical sensing.

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INTRODUCTION New materials and structures that effectively confine light to small volumes are central for the development and advancement of nanophotonic applications such as quantum communication and computing1, nanolasers2, ultra-small photonic filters3, and optical sensing.4 Photons possess no charge or rest mass and are prone to escaping when trapped in photonic structures.5,6 The confinement of light within small volumes comparable to the wavelength of light is challenging. However, high quality optical microcavities with strong light confinement capabilities to attain the precise control of light emission and propagation have been realized.7,8 Photonic crystals (PCs) have emerged as the most promising platforms to develop efficient optical microcavities with high quality factor and small cavity volume.9,10 Optical microcavities (µCVs) are photonic crystal structures that can guide and build up optical signals by light confinement.6 Typically, µCVs consists of two plane-parallel mirrors positioned apart at a fixed distance with the objective of capturing and storing light indefinitely, until the system is triggered to release the confined light from the cavity in a controlled fashion.6,11,12 µCVs can be produced in different materials, including polymers13, semiconductors such as GaAs, InP, GaInAsP and GaN14-17 and silicon4,18,19. Usually, µCVs are fabricated by a combination of lithographic and etching techniques, and chemical or physical vapor deposition.17 However, alternative materials such as porous silicon produced by electrochemical etching of silicon opened new opportunities to develop nanoporous µCVs with tunable optical properties and nanoporous architectures for different applications, including light-emitting devices, solar cells, optical filters, biosensors, drug delivery and theranostics.20-22 The modulation of porosity in depth by the anodizing current allows the effective engineering the optical properties of porous silicon µCVs, the composite air-silicon matrix of which acts as a versatile effective medium.23-27 Porous silicon has outstanding optoelectronic properties, however it has poor chemical stability and mechanical strength and 2 ACS Paragon Plus Environment

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its fabrication process requires the use of extremely hazardous HF-based electrolytes.28,29 To date, different alternative/complementary nanoporous materials have been explored to overcome the intrinsic limitations of porous silicon. Of these, nanoporous anodic alumina (NAA) produced by anodization of aluminum has superior properties to those of porous silicon in respect of mechanical, thermal and chemical stabilities as well as versatile nanopore geometry. Furthermore, anodization of aluminum is a well-established electrochemical process performed in mild acid electrolyte solutions that is economical and fully scalable process and requires minimum safety measures.30-36 Recent studies have demonstrated that the effective refractive index of NAA can be precisely modulated in a multi-dimensional fashion to create a broad range of PC structures with finely tuned optical properties (e.g. distributed Bragg reflectors, gradient-index filters, bandpass and linear variable bandpass filters, encoded photonic tags, etc.). The realization of NAA-based µCVs has been demonstrated in a few pioneering studies.37-40 The fabrication of NAA-µCVs involves the introduction of defect modes in the PC structure, which can be achieved by various approaches such as the insertion of a thin layer of nanopores with constant effective refractive index between two highly reflective Bragg mirrors, a phase shift of effective refractive index between Bragg mirrors, or a progressive asymmetric modulation of the effective medium in depth.37,38,41 However, the maximum quality factors of NAAµCVs reported by Wang et al. (~24)38, Lee et al. (~55)37 and Yan et al. (~45)41 were found to be significantly lower than those of porous silicon-based µCVs (~1500-3400)19,42,43 due to the low refractive index of alumina (Al2O3 – nAlumina~1.70).37-40 Despite this limitation, the development of new pulse-like anodization strategies and novel NAA-PC architectures provides new opportunities to improve the quality of NAA-µCVs and explore new strategies to attain strong light confinements by a precise control of light–matter interactions at the nanoscale.37-59 Recently, we identified sharp resonance bands within the photonic stopband 3 ACS Paragon Plus Environment


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(PSB) of NAA-PCs produced by stepwise pulse anodization when a logarithmic negative apodization function was applied under certain conditions.60 Motivated by these results, we decided to explore this nanofabrication approach to develop high-quality NAA-µCVs.

Figure 1. Fabrication of NAA-µCVs by apodized stepwise pulse anodization (ASPA). a) Representative ASPA profile (left) and structure of NAA-µCVs (right) showing details of the existing relationship between nanopore geometry and effective refractive index (neff) distribution between high (alumina) and low (air) values along the nanopore depth. b) Schematic showing the confinement of light within the structure of NAA-µCVs (left) and representative transmission spectra showing the characteristic photonic stopband (PSB) and the resonance band at its center in NAA-µCVs. Insets show a digital picture of that NAA-µCVs with the characteristic green interferometric color denoting the position of the photonic stopband and a magnified view of the red rectangle showing details of the resonance band (note: NAA-µCV produced with TP = 1300 s, ∆AJ = 0.210 mA cm-2, JOffset = 0.280 mA cm-2, tAn = 25 h, and tpw = 6 min).

In this study, a new architecture of NAA-µCVs that enables high quality confinement of light by means of a rationally designed apodized stepwise pulse anodization (ASPA) approach is presented. A negative apodization function is implemented into the stepwise pulse anodization profile with the aim of modulating the effective refractive index of NAA in depth and engineering the photonic stopband (PSB) of the Bragg mirrors (Figure 1).61,62 Various anodization parameters including anodization time, anodization period, current density offset, and pore widening time are systematically modified in order to maximize and 4 ACS Paragon Plus Environment

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tune the resonance band of NAA-µCVs across the spectral regions. Our study establishes a comprehensive rationale towards the fabrication of NAA-µCVs with high quality factor and optimized optical properties. These PCs will enable new opportunities to expand the applicability of NAA-µCVs across disciplines such as ultra-sensitive sensors, light harvesting/emitting devices, and optical filters. EXPERIMENTAL SECTION 2.1. Materials. High purity (99.9997%) aluminum (Al) foils of thickness 0.32 mm were supplied by Goodfellow Cambridge Ltd. (UK). Sulfuric acid (H2SO4), perchloric acid (HClO4), copper (II) chloride (CuCl2), hydrochloric acid (HCl), phosphoric acid (H3PO4), and ethanol (EtOH-C2H5OH) were supplied by Sigma-Aldrich (Australia) and used as received, without further purification steps. Aqueous solutions used in this study were prepared with Milli-Q® water (18.2 mΩ·cm). 2.2. Fabrication of Nanoporous Anodic Alumina Microcavities. NAA optical microcavities (NAA-µCVs) were produced by apodized stepwise pulse anodization (ASPA) under current density control conditions. 1.5 x 1.5 cm Al square chips were cleaned under sonification in EtOH and Milli-Q® water for 15 min each, then dried under air stream. Prior to anodization, these Al substrates were electropolished in a mixture of EtOH and HClO4 4:1 (v:v) at 20 V and 5°C for 3 min. The anodization process of Al substrates was carried out in an electrochemical reactor at a constant temperature of -1°C, using an aqueous solution of 1.1 M H2SO4 with 25 v% of EtOH as electrolyte. The galvanostatic anodization process started with a constant current density step at 1.120 mA cm-2 for 1 h to create a starting nanoporous oxide layer that acts a shuttle for achieving a homogenous pore growth rate preceding the ASPA step. The anodization profile was subsequently set to apodized stepwise pulse mode. A logarithmic negative apodization function was implemented into conventional stepwise pulse



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anodization. The current density-time ASPA profiles were produced by a custom-designed Labview®-based software according to Equation 1. (1)

where AJ (t) is the time-dependent current density amplitude, which follows a logarithmic negative apodization function as defined in Equations 2 and 3. For t ≤ tAn/2

(2)

For t > tAn/2

(3)

where Amax and Amin are the maximum and minimum amplitudes and tAn is the total anodization time at ASPA. Note that TP in the ASPA profile was defined as the total time length of high and low anodization current density pulses (Equation 4): (4)

where thigh and tlow are the time duration at high and low current density values, respectively. The ratio between thigh and tlow was set to thigh:tlow = 1:4. To gain a better understanding of the light-confining capabilities of NAA-µCVs produced by ASPA, different anodization parameters (i.e. anodization time –tAn, anodization period –TP, current density offset –Joffset, and pore widening time –tpw) were systematically manipulated


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to assess their effects on the optical characteristics of NAA-µCVs, such as interferometric color, position of the resonance band (λR), full-width at half maximum of the resonance band (FWHMR) and quality factor of cavity (QC), as defined by Equation 5. (5)

2.3. Optical Characterization. Prior to optical characterization, the remaining aluminum substrate was selectively dissolved from the backside of these aluminum chips by wet chemical etching in a saturated solution of HCl/CuCl2, using an etching cell with a Viton® mask with a circular window of 5 mm in diameter. These etched NAA-µCVs were then optically characterized. The optical transmission spectra of NAA-µCVs fabricated at various conditions were obtained at normal incidence (i.e. θ = 0°) from 200–1000 nm with a resolution of 1 nm and 5 mm slit using a UV-vis-NIR spectrophotometer (Cary 60, Agilent, USA), and from 200–1500 nm with a resolution of 1 nm in a UV-vis-NIR spectrometer (UV3600 Plus, Shimadzu, Japan). The interferometric color displayed by these NAA-µCVs as a function of the different fabrication parameters was characterized through digital images acquired by a Canon EOS 700D digital camera equipped with a Tamron 90 mm F2.8 VC USD macro mount lens with autofocus function under natural illumination. A black card was used as background for the digital image acquisition. The pore size of NAA-µCVs was widened by isotropic chemical etching in an aqueous solution of 5 wt% H3PO4 at 35°C at different pore widening times (i.e. tpw = 0, 2, 4, and 6 min) and their transmission spectra and digital images were recorded after each pore widening step. Note that the features of the resonance band of these NAA-µCVs (i.e. position of resonance band –λR, full-width at half maximum of the resonance band –FWHMR, and baseline of resonance band – y0) were estimated using OriginPro 8.5®, applying Gaussian fittings over the resonance bands shown in the transmission spectra of NAA-µCVs and using as a baseline the lower lobe of the 7 ACS Paragon Plus Environment


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photonic stopband (PSB), as illustrated in Figure 1. The obtained results were summarized in contour maps generated using OriginPro 8.5®, using a triangulation algorithm in which the coordinates of the intersection point were computed with linear interpolation. 2.4. Structural Characterization. The nanoporous structure of NAA-µCVs was characterized by a field emission gun scanning electron microscope (FEG-SEM FEI Quanta 450). The obtained FEG-SEM images were analyzed using ImageJ (public domain program developed at the RSB of the NIH).63 RESULTS AND DISCUSSION 3.1. Fabrication and Structural Characterization of Nanoporous Anodic Alumina Microcavities. Figure 1a illustrates the fabrication process of NAA-µCVs by ASPA. The structure of these PC structures is composed of two highly reflective distributed Bragg reflector (DBR) mirrors with asymmetrically modulated effective refractive index in a stepwise fashion in depth following a logarithmic negative window. The amplitude of the current density pulse is logarithmically reduced during the fabrication process of the first half of the NAA-µCV (i.e. from t = 0 to t = tAn/2) (Equation 2). At t = tAn/2, the current density amplitude is progressively increased according to the mathematical expression shown in Equation 3. The transmission spectrum of these PC structures is characterized by a relatively broad PSB with a strong and narrow resonance band at approximately its central position (Figure 1b). This resonance band denotes a strong confinement of light within the NAAµCV at that narrow range of wavelengths, which is established by the geometric features of the NAA-DBR mirrors. Figures 2a-c show a set of representative FEG-SEM images of NAA-µCVs produced by ASPA. These images reveal that the structure of these PCs is composed of stacked layers of NAA with a porosity modulation in depth that follows the ASPA current density profile applied during the anodization process. Figure 2a depicts a


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representative top view FEG-SEM image of a NAA-µCV, revealing nanopores with an average pore diameter (dp) of 15 ± 2 nm that are randomly distributed across the surface.

Figure 2. Nanoporous structure of NAA-µCVs produced by ASPA. a) Representative top view FEG-SEM image of a NAA-µCV showing a random distribution of nanopores with dP = 15 ± 2 nm (scale bar = 500 nm). b) General cross-sectional view FEG-SEM image of a NAA-µCV showing a top layer of straight nanopore diameter (constant anodization = CA – thickness = 1.4 ± 0.1 µm) and the PC microcavity layer with nanopore dimeter modulation (ASPA – thickness = 11.0 ± 0.1 µm) with details showing the areas where the period length (LTP) was estimated (i.e. from 0-2 µm to 8-10 µm) (scale bar = 5 µm). c) Magnified view of the white rectangle shown in (b) revealing the porosity modulation in depth in NAA-µCVs (scale bar = 1 µm). d) Magnified views at different cross-sectional positions (i.e. 0-2, 4-6, and 8-10 µm) showing details of the period length modulation (scale bar = 500 nm). e) Illustration representing the idealized nanoporous structure of NAA-µCVs, where the period length (LTP) is modified in depth to create an optical microcavity structure with a logarithmically modulated effective refractive index composed of two NAA-DBRs with asymmetrically modulated effective refractive index. f) Bar chart showing the period length distribution in NAA-µCVs produced by ASPA along the nanopore depth (note: NAA-µCV produced with TP = 800 s, ∆AJ = 0.210 mA cm-2, JOffset = 0.280 mA cm-2, tAn = 20 h, and tpw = 6 min).



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Figure 2b shows a general cross-sectional view of a NAA-µCV produced with TP = 800 s, Joffset = 0.280 mA cm-2, ∆AJ = 0.210 mA cm-2, tAn = 10 h, and tpw = 6 min. That image reveals that the structure of these NAA-µCVs is composed of a top layer 1.4 ± 0.1 µm thick with constant nanopore diameter, which results from the first anodization stage at constant anodizing current density (CA – J = 1.120 mA cm-2 for 1 h), and a much thicker layer (i.e. 11.0 ± 0.1 µm) with nanopore diameter modulation in depth corresponding to the ASPA stage (Figures 2c and d). A pore branching effect can be observed at the bottom part of the NAA-µCV structure (Figure 2d; 8-10 µm), which can be associated with the non-selforganization conditions used in our study. A closer analysis of the period length (i.e. LTP – distance between adjacent layers in the ASPA section of the NAA-µCVs) reveals a direct dependency with the anodization amplitude (∆AJ) (Figure 2e). For instance, we estimated the average LTP along the ASPA cross-section of a NAA-µCV produced with TP = 800 s, Joffset = 0.280 mA cm-2, ∆AJ = 0.210 mA cm-2, tAn = 10 h, and tpw = 6 min every 2 µm by FEG-SEM image analysis. Our results indicate that LTP varies along the nanopore depth following the apodization function applied during anodization, with an estimation of 115 ± 9 nm at 0–2 µm, 112 ± 6 nm at 2–4 µm, 96 ± 8 nm at 4–6 µm, 111 ± 6 nm at 6–8 µm, and 113 ± 5 nm at 8–10 µm (Figure 2f). From this analysis it is apparent that LTP is reduced at the center of the ASPA section of the NAA-µCVs (i.e. t ~ tAn/2), which is expected due to the significant reduction of the anodization amplitude at t = tAn/2. Furthermore, FEG-SEM image analysis denote a modulation of porosity in depth with the pulse anodizing current density. For instance, the porosity at J = 0.280 and 1.120 mA cm-2 (i.e. minimum and maximum values of current density) was estimated to be 18 ± 9 and 32 ± 13%, respectively.48 Apodization is a filtering technique broadly used in optics to narrow the PSB of photonic structures. The direct relationship between the geometric features of NAA-µCVs and the anodization parameters


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enables the apodization of the optical signals of these PCs by structural engineering of their effective medium (i.e. effective refractive index modulation). 3.2. Effect of Anodization Time (tAn) on the Optical Properties of Nanoporous Anodic Alumina Microcavities. To understand the effect of tAn on the optical characteristics of NAA-µCVs, a set of NAA-µCVs was fabricated using logarithmic negative ASPA with varying tAn from 5 to 25 h at an interval of 5 h. Other anodization parameters such as anodization period (TP), current density offset (Joffset), and amplitude difference (∆AJ) were fixed at 1200 and 1300 s, 0.280 mA cm-2, and 0.210 mA cm-2, respectively. Figures 3a and b show representative anodization profiles for NAA-µCVs fabricated at different tAn (i.e. 5, 10, 15, 20, and 25 h) for TP = 1200 and 1300 s, respectively. These ASPA profiles denote that, under the anodization conditions used in our study, the anodizing current density profile is precisely translated into modulations of voltage throughout the whole process, resulting in an internal modulation of dp in depth. The transmission spectra of NAA-µCVs produced at TP = 1300 s as a function of tAn and pore widening time (tpw) are displayed in Figures 4a-e. The optical properties of these NAA-µCVs were characterized in terms of λR, FWHMR, Qc, and interferometric color. Figure 4 shows that the PSB of these NAA-µCVs is located within visible-NIR range and it undergoes a blue shift and increases its intensity and width with tpw, from 0 to 6 min. The interferometric color is a result of the selective and constructive reflection of light by the NAA-µCV structure and denotes the position of the PSB within the UV (transparent), visible (color), or NIR (transparent) spectral regions. The transmission spectra of these NAA-µCVs shows a resonance band located at approximately the center of the PSB, which indicates the presence of an optical microcavity within the structure of these PCs.



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Figure 3. Representative anodization profiles of NAA-µCVs produced by ASPA at different anodization times and anodization periods (note: NAA-µCVs produced with TP = 1200 and 1300 s, ∆AJ = 0.210 mA cm-2 and JOffset = 0.280 mA cm-2). a) Anodization profiles of NAA-µCVs produced with TP = 1300 at tAn = 5, 10, 15, 20, and 25 h. b) Anodization profiles of NAA-µCVs produced with TP = 1200 at tAn = 5, 10, 15, 20, and 25 h.

Figures 5a-e show magnified views of the resonance bands observed in the transmission spectra of these NAA-µCVs (Figures 4a-e) with details of Gaussian fittings used to estimate λR, FWHMR, and Qc. In general, it can be observed that the resonance band of these NAA-


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µCVs rises as tpw increases, becoming more well-resolved and intense due to the effective refractive index contrast enhancement between adjacent NAA layers.

Figure 4. Combinational effect of anodization time (tAn) and pore widening time (tpw) on the transmission spectrum of NAA-µCVs produced by ASPA at TP = 1300 s (note: color rectangles denote the approximate position of the resonance band within the PSB and black dotted arrow lines indicate the blue shift of the resonance bands with tpw). a) tAn = 5, b) tAn = 10, c) tAn = 15, d) tAn = 20, and e) tAn = 25 h. Insets in a-e display digital pictures of these photonic crystal structures showing vivid interferometric colors when the resonance band is located within the visible region and transparent when the band is within the UV or NIR spectral regions.



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Figure 5. Combinational effect of anodization time (tAn) and pore widening time (tpw) on the resonance band of NAA-µCVs produced by ASPA at TP = 1300 s (note: horizontal dotted black lines denote the baseline (y0) used for the Gaussian fittings, which correspond to the lower lobe of the PSB, and vertical dotted red lines indicate the central wavelength of the resonance band (λR) and the symmetry of the Gaussian fitting). a) tAn = 5, b) tAn = 10, c) tAn = 15, d) tAn = 20, and e) tAn = 25 h.


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The stepwise modulation of nanopores within NAA-µCVs is more evident as the nanoporous structure of NAA is chemically etched, since those layers produced at lower anodizing current density dissolve at a faster rate than those produced at higher J (Figure S1 – Supporting Information). However, the over-etching of the NAA-µCVs’ structure (tpw > 4 min) results in a broadening of the resonance band and a decrement of its intensity due to light scattering by the overall PC structure (Figure S2 – Supporting Information). So, for NAA-µCVs produced at tAn = 5, 10, and 15 h, the resonance band is almost vanished from the transmission spectrum at tpw = 6 min. To further extend the analysis on the effect of tAn on the optical properties of NAA-µCVs produced by ASPA, we fabricated another set of NAAµCVs under the same conditions (i.e. Joffset = 0.280 mA cm-2, ∆AJ = 0.210 mA cm-2, and tAn = 5, 10, 15, 20, and 25 h) but setting the anodization period at TP = 1200 s. Figure 3b shows representative anodization profiles of these NAA-µCVs produced at TP = 1200 s, showing how the anodizing current density (input) is directly translated into voltage (output) changes in a dynamic fashion, without apparent delay. Figures 6a-e show the transmission spectra of NAA-µCVs produced with TP = 1200 s, which also include digital pictures displaying the characteristic interferometric color of these PC structures. As these graphs indicate, the PSB of these NAA-µCVs is located within the visible-NIR region of the spectrum, although slightly blue-shifted as compared to their TP = 1300 s counterparts. As demonstrated in previous studies, this blue shift is associated with the reduction of the anodization period, which results in a shorter period length (LTP) within the nanoporous structure of NAAµCVs.43-59 The transmission spectra of these NAA-µCVs displays a PSB that increases its intensity with tpw and a well-defined resonance band at the center of the PSB. Figures 7a-e compile magnified views of the resonance bands observed in the transmission spectra of these NAA-µCVs (Figures 6a-e), with details of Gaussian fittings used to estimate λR, FWHMR, and Qc. 15 ACS Paragon Plus Environment


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Figure 6. Combinational effect of anodization time (tAn) and pore widening time (tpw) on the transmission spectrum of NAA-µCVs produced by ASPA at TP = 1200 s (note: color rectangles denote the approximate position of the resonance band within the PSB and black dotted arrow lines indicate the blue shift of the resonance bands with tpw). a) tAn = 5, b) tAn = 10, c) tAn = 15, d) tAn = 20, and e) tAn = 25 h. Insets in a-e display digital pictures of these photonic crystal structures showing vivid interferometric colors when the resonance band is located within the visible region and transparent when the band is within the UV or NIR spectral regions.

The quality factor (QC), defined as the ratio of the resonance band wavelength (λR) to its full width at half maximum (FWHMR) (Equation 5), is an important criteria in assessing the strength of photon confinement within optical microcavities.64,65 The QC of these NAA-µCVs was estimated by fitting the resonance bands shown in Figures 5 and 7 to Gaussian envelopes, using as a baseline the lower love of the PSB as indicated by the horizontal dotted black lines showed in these graphs. 16 ACS Paragon Plus Environment

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Figure 7. Combinational effect of anodization time (tAn) and pore widening time (tpw) on the resonance band of NAA-µCVs produced by ASPA at TP = 1200 s (note: horizontal dotted black lines denote the baseline (y0) used for the Gaussian fittings, which correspond to the lower lobe of the PSB, and vertical dotted red lines indicate the central wavelength of the resonance band (λR) and the symmetry of the Gaussian fitting). a) tAn = 5, b) tAn = 10, c) tAn = 15, d) tAn = 20, and e) tAn = 25 h.



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A summary of the estimated values of y0, λR, FWHMR and QC are compiled in Tables S1-S3 (Supporting Information) along with the quality (R2) of these Gaussian fittings. Figure 8 shows contour maps summarizing the dependency of QC and λR with tAn and tpw for NAAµCVs produced with TP = 1200 and 1300 s, Joffset = 0.280 mA cm-2, and ∆AJ = 0.210 mA cm-2. Figure 8a shows the dependency of QC on tAn and tpw for NAA-µCVs fabricated with TP = 1300 s. It is apparent that QC becomes more dependent on these fabrication parameters at short pore widening times (i.e. from tpw = 0 to 2 min) and longer anodization times (i.e. from tAn = 15 to 25 h). This trend is denoted by a closer and denser concentration of color fields around the QC maximum (i.e. QC = 63.1 ± 1.2), which is located at tpw = 2 min and tAn = 20 h. In general, an increase in tAn leads to an enhancement of the QC of NAA-µCVs, while longer pore widening times worsen the quality factor of the NAA-µCVs. The relationship between

λR with tAn and tpw for NAA-µCVs produced with TP = 1300 s is displayed in Figure 8b, where it can be clearly observed that the distance between color fields is closer as tAn is reduced from 10 to 5 h. This indicates a strong dependency of λR with tAn at shorter anodization times. Furthermore, it is observed that λR is blue-shifted with tpw across the UVvisible spectrum from tAn = 5 to 25 h. This analysis also reveals that the effect of tAn on the position of the resonance band is not as significant as that of tpw, since only a slight red shift is observed as tAn increases from 5 to 20 h, and a slight blue shift from 20 to 25 h, achieving its maximum value (i.e. 725 ± 1 nm) at tAn = 5 h and tpw = 0 min. The distribution of Qc and λR for NAA-µCVs produced at TP = 1200 s with tAn and tpw is presented in Figures 8c and d, respectively. The contour map shown in Figure 8c reveals a concentration of color fields at the region of longer tAn and shorter tpw, achieving a local maximum of QC (i.e. 112.6 ± 5.2) at tAn = 20 h and tpw = 2 min, which is the highest quality factor reported for a NAA-based optical microcavity to date.


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Figure 8. Combinational effect of anodization time (tAn) and pore widening time (tpw) on the optical properties of NAA-µCVs (i.e. quality factor – QC, position of resonance band – λR, and interferometric color) produced by ASPA. a) Contour map showing the dependency of QC with tAn and tpw for NAA-µCVs produced with TP = 1300 s. b) Contour map showing the dependency of λR with tAn and tpw for NAA-µCVs produced with TP = 1300 s. c) Contour map showing the dependency of QC with tAn and tpw for NAA-µCVs produced with TP = 1200 s. d) Contour map showing the dependency of λR with tAn and tpw for NAA-µCVs produced with TP = 1200 s. e) Digital images showing the interferometric color displayed by NAA-µCVs produced with TP = 1200 s as a function of tAn and tpw. f) Digital images showing the interferometric color displayed by NAA-µCVs produced with TP = 1300 s as a function of tAn and tpw.



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The distance between field lines around the local maximum is shorter, indicating a stronger dependency of QC on tAn and tpw around that set of fabrication conditions. In contrast, the combination of shorter tAn (i.e. 5 to 15 h) and longer tpw (i.e. 4 to 6 min) worsens QC as the distance between field lines and color fields is relatively broad at these areas. Figure 8d shows how tAn and tpw affect λR of NAA-µCVs produced at TP = 1200 s. This graph denotes a homogenous but broad distribution of color fields with equidistant field lines throughout, from tAn = 5 to 25 h and from tpw = 0 to 4 min, which indicates a weak dependency of λR on tAn and tpw at these combinations of fabrication parameters. As tpw increases from 4 to 6 min and tAn decreases from 10 to 5 h, λR shows a stronger dependency with these parameters, as suggested by the closer field lines and high density of color fields. λR is red-shifted as tAn increases from 5 to 20 h and slightly blue-shifted when tAn increases from 20 to 25 h. The maximum value of λR (i.e. 665 ± 1 nm) is achieved at tAn = 20 h and tpw = 2 min. Furthermore, this analysis also reveals that tpw blue-shifts the resonance band of NAA-µCVs produced at TP = 1200 s as tpw increases. The comparative analysis of Figures 8a and c reveals that a combination of long tAn (i.e. from 15 to 25 h) and short tpw (i.e. from 0 to 2 min) results in high quality NAA-µCVs with narrow and well-resolved resonance bands. The average QC estimated for NAA-µCVs produced with TP = 1200 and 1300 s were 46.4 ± 24.8 and 33.7 ± 13.8, respectively. It is worthwhile noting that only one of the NAA-µCVs produced at 1300 s showed superior light-confining performance than those reported in previous studies (i.e. QC = 63.1 ± 1.2 for NAA-µCV produced with TP = 1300 s, tpw = 2 min and tAn = 20 h – Wang et al. (~24)38, Lee et al. (~55)37 and Yan et al. (~45)41). However, up to six NAA-µCVs produced with TP = 1200 s showed superior properties to confine light than previously NAAbased optical microcavities (i.e. QC = 64.0 ± 1.0 at tpw = 2 min and tAn = 10 h, QC = 70.5 ± 2.1 at tpw = 4 min and tAn = 10 h, QC = 66.0 ± 1.3 at tpw = 6 min and tAn = 15 h, QC = 75.2 ± 3.1 at tpw = 0 min and tAn = 20 h, QC = 112,6 ± 5.2 at tpw = 2 min and tAn = 20 h, and QC = 73.5 ± 2.6 20 ACS Paragon Plus Environment

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at tpw = 4 min and tAn = 20 h). Although both types of NAA-µCVs have a weak correlation between the shift in λR with tAn, Figures 8b and d show that the λR of NAA-µCVs produced at TP = 1300 s evolves in a slightly different manner with tAn as compared to NAA-µCVs fabricated at TP = 1200 s. This graph also indicates that, as observed in the transmission spectra (Figures 4 and 6) the position of the PSB and resonance bands of NAA-µCVs produced at TP = 1200 s is blue-shifted as compared to their equivalent NAA-µCVs fabricated at TP = 1300 s. It is also observed that both sets of NAA-µCVs show a stronger dependency of λR with tpw than that shown for tAn. A pore widening treatment blue-shifts the position of the respective resonance bands, and the longer tpw is, the shorter the wavelength where the resonance band is positioned at. It is worthwhile to note that, for a given tpw, the position of the resonance band of NAA-µCVs at TP = 1300 s is located at longer wavelengths than that of NAA-µCVs produced with TP = 1200 s due to the red shift associated with the longer anodization period and longer period length. Another interesting optical property of NAA-µCVs is their vivid interferometric colors, which correspond to the wavelength of their respective PSB and λR when these are positioned within the visible range of the spectrum. Figures 8e and f compile digital images of NAA-µCVs produced at TP = 1200 and 1300 s as a function of tAn and tpw, respectively. The analysis of these images is in good agreement with the results obtained in Figures 8b and d, where the λR is blue-shifted with tpw. It is also apparent that λR is red-shifted with increasing tAn from 5 to 20 h and slightly blue shifted from tAn = 20 to 25 h. 3.3. Effect of Anodization Period (TP) on the Optical Properties of Nanoporous Anodic Alumina Microcavities. To demonstrate the tuneability of the position of the resonance band across the spectral regions and to further optimize the quality of NAA-µCVs produced by ASPA, we produced a set of NAA-µCVs with different TP, where this fabrication parameter 21 ACS Paragon Plus Environment


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was systematically modified from 800 to 1300 s with ∆TP = 100 s, while keeping constant Joffset and ∆AJ at 0.280 mA cm-2 and 0.210 mA cm-2, respectively. The anodization profiles and transmission spectra of these NAA-µCVs are shown in Figure S3 (Supporting Information) and Figure 9, respectively. Figures 9a-d show the transmission spectra of these NAA-µCVs as a function of TP and tpw (i.e. from 0 to 6 min) and Figure 10 shows magnified views of the resonance bands and Gaussian fittings used to estimate λR, FWHMR and QC. In all these cases it is verified that the PSB and resonance band of theses NAA-µCVs is red-shifted with TP and blue shifted with tpw. Note that those NAA-µCVs produced with TP > 800 s also showed second and third order PSBs. NAA-µCVs fabricated with TP = 900 s have both first and second order PSBs, while NAA-µCVs produced at TP = 1000, 1100, 1200 and 1300 s show second and third order PSBs. However, the first order PSB plays the primary role in determining the optical properties of NAA-µCVs since this band is much more intense and well-resolved than their higher order counterparts. At tpw = 0 min (Figure 9a), all NAA-µCVs display a weak resonance band within their first order PSB, which is in the range of 400 to 700 nm and red-shifted with TP. As TP increases, the intensity of the resonance band increases and shifts its position (λR) toward the NIR spectral region. As the digital pictures shown in Figures 9a-d (insets) indicate, for a given tpw the interferometric color of these NAA-µCVs is red-shifted with TP. For instance, at tpw = 0 min, the interferometric color of NAA-µCVs changes from transparent (i.e. UV region) (TP = 800 s), blue (TP = 900 s), green (TP = 1000 s), orange (TP = 1100 s) to transparent (i.e. NIR region) (TP = 1200 and 1300 s) as TP increases. It is also verified that the resonance band increases its intensity and it is blue-shifted with a pore widening treatment (i.e. tpw increases), which is in good agreement with our previous observation.


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Figure 9. Combinational effect of anodization period (TP) and pore widening time (tpw) on the optical properties of NAA-µCVs (i.e. quality factor – QC, position of resonance band – λR, and interferometric color) produced by ASPA. a-d) Transmission spectra showing the position of the resonance band (colored rectangles) and digital pictures (insets) of NAA-µCVs for each anodization period (TP = 800–1300 s) at different pore widening times (i.e. a) tpw = 0 min, b) tpw = 2 min, c) tpw = 4 min, and d) tpw = 6 min).



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Figure 10. Combinational effect of anodization period (TP) and pore widening time (tpw) on the resonance band of NAA-µCVs produced by ASPA (note: horizontal dotted black lines denote the baseline (y0) used for the Gaussian fittings, which correspond to the lower lobe of the PSB, and vertical dotted red lines indicate the central wavelength of the resonance band (λR) and the symmetry of the Gaussian fitting). a) tpw = 0 min. b) tpw = 2 min. c) tpw = 4 min. d) tpw = 6 min. e) Contour map showing the dependency of QC with TP and tpw for NAAµCVs produced with TP = 800–1300 s. f) Contour map showing the dependency of λR with TP and tpw for NAAµCVs produced with TP = 800–1300 s.


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Figures 10e and f show contour maps depicting the graphical correlation of QC and λR of NAA-µCVs with TP and tpw (Table S2). Figure 10e reveals that the color fields in the region of shorter TP (i.e. from 800 to 1000 s) are relatively broad across tpw, from 0 to 6 min, which is a visual indication of the weak correlation of QC with the fabrication parameters for NAAµCVs produced at shorter TP. Therefore, NAA-µCVs produced at TP = 800, 900 and 1000 s have low QC. In contrast, the color fields become denser with closer field lines when TP increases from 1100 to 1300 s. This suggests that QC has a stronger dependency with longer TP, where the maximum of QC (i.e. 112.6 ± 5.2) is achieved at TP = 1200 s and tpw = 2 min. The average QC estimated for NAA-µCVs as a function of TP, excluding those PCs without resonance band, was 44.5 ± 23.1. However, seven of these NAA-µCVs showed superior light-confining performance than those reported in previous studies (i.e. QC = 56.8 ± 3.2 at TP = 900 s and tpw = 4 min, QC = 60.2 ± 2.1 at TP = 1000 s and tpw = 0 min, QC = 56.5 ± 1.5 at at TP = 1100 s and tpw = 2 min, QC = 75.2 ± 3.3 at TP = 1200 s and tpw = 0 min, QC = 112.6 ± 5.2 at TP = 1200 s and tpw = 2 min, QC = 73.5 ± 4.1 at TP = 1200 s and tpw = 4 min, and QC = 63.1 ± 1.2 at TP = 1300 s and tpw = 2 min). The distribution of λR as a function of TP and tpw is depicted in Figure 10f. This graph shows two local minima in the contour plot due to the absence of resonance bands within the PSB of these NAA-µCVs, which are located at tpw = 0 min for TP = 1100 s as well as at tpw = 6 min for TP = 800 s. At tpw = 2 and 4 min, the color distribution reveals a red shift in λR resulting from the manipulation of TP from 800 to 1300 s, where the longest resonance wavelength is achieved at TP = 1300 s and tpw = 0 min (i.e. 709 ± 1 nm). In general, the longer the anodization period, the longer the wavelength at which NAA-µCVs confine light. This analysis also indicates that an increase in tpw results in a blue shift of λR, thus NAA-µCVs confine light of shorter wavelengths although in a less efficient manner as indicated by the QC analysis shown in Figure 10e due to light scattering effect.



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3.4. Effect of Current Density Offset (Joffset) on the Optical Properties of Nanoporous Anodic Alumina Microcavities. To further understand the effect of the different fabrication parameters on the photonic features of NAA-µCVs produced by ASPA, we investigated the how the current density offset (Joffset) affects the quality factor and the tuning of resonance bands of NAA-µCVs. To this end, Joffset was systematically modified from 0.140 to 0.560 mA cm-2 with an interval of 0.140 mA cm-2 while keeping constant the rest of fabrication parameters (i.e. TP = 1300 s, Joffset = 0.280 mA cm-2, and ∆AJ = 0.210 mA cm-2). The anodization profiles of NAA-µCVs produced at different Joffset are compiled in Figure S4 (Supporting Information). The transmission spectra of these NAA-µCVs shown in Figures 11a-d were analyzed to establish the effect of this fabrication parameter on QC and λR. Figure 11a shows the transmission spectra of NAA-µCVs produced at Joffset = 0.140 mA cm-2 as a function of tpw (i.e. 0 to 6 min). The PSB of this set of NAA-µCVs is located within the visible region, with a very weak resonance band that is almost vanished at long pore widening times (i.e. tpw > 2 min). The pore widening treatment blue-shifts the position of the PSB and leads the NAA-µCV to lose its light-confining characteristics. The transmission spectra shown in Figure 11b reveals that the position of the PSB of NAA-µCVs produced at Joffset = 0.280 mA cm-2 are located within the upper range of visible spectrum (i.e. 600–800 nm). Unlike NAA-µCVs produced at Joffset = 0.140 mA cm-2, the resonance band of these NAA-µCVs (Joffset = 0.280 mA cm-2) remains well-resolved and sharp after pore widening from 0 to 6 min. On the other hand, both sets of NAA-µCVs produced at Joffset = 0.420 and 0.560 mA cm-2 have their PSBs located in the NIR range (i.e. 900–1100 nm) (Figures 11c and d). These NAA-µCVs also show the presence of an intense resonance band within their PSB, which is slightly widen and blue-shifted with the pore widening treatment, from 0 to 6 min.


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Figure 11. Combinational effect of anodization offset (JOffset) and pore widening time (tpw) on the optical properties of NAA-µCVs (i.e. quality factor – QC, position of resonance band – λR, and interferometric color) produced by ASPA. a-d) Transmission spectra showing the position of the resonance band, digital pictures (insets) of NAA-µCVs for each anodization offset (JOffset = 0.140–0.560 mA cm-2) at different pore widening times (from 0 to 6 min) (i.e. a) JOffset = 0.140 mA cm-2, b) JOffset = 0.280 mA cm-2, c) JOffset = 0.420 mA cm-2, and d) JOffset = 0.560 mA cm-2) (note: color rectangles denote the approximate position of the resonance band within the PSB and black dotted arrow lines indicate the blue shift of the resonance bands with tpw).



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By comparing the transmission spectra at different Joffset, it is apparent that an increase in Joffset causes a red shift in the position of resonance band. As the digital images shown in the insets in Figures 11a-d demonstrate, these NAA-µCVs display vivid interferometric colors, which are affected by the fabrication parameters: Joffset and tpw. Although these NAA-µCVs show second and third order PSBs, the color displayed by these PCs corresponds to the wavelength at which the first order PSB is located, denoting a more efficient reflection of light within these spectral regions. In this case, NAA-µCVs produced at lower Joffset (i.e. 0.140 and 0.280 mA cm-2) display vivid colors corresponding to the position of their PSB in the visible spectral range. In contrast, the PSBs of NAA-µCVs produced with high Joffset (i.e. 0.420 and 0.560 mA cm-2) are within the NIR range, thus no color is observed (i.e. transparent – black). Figures 12a-d show magnified views of the resonance bands and Gaussian fittings used to estimate λR, FWHMR and QC for these NAA-µCVs and Figures 12e and f compile a summary of the estimated values for QC and λR in the form of contour maps. The visual analysis of the magnified resonance bands shown in Figures 12a-d reveals that, in general, the intensity of the resonance band increases with JOffset and decreases with tpw. A closer analysis of the values of QC, visually shown in Figure 12e and compiled in Table S3 (Supporting Information), reveals that the combination of low values of Joffset (e.g. 0.1400.280 mA cm-2) and moderate pore widening times (i.e. 2-4 min) are favorable in the production of NAA-µCVs with high quality resonance bands. The QC maximum is achieved by NAA-µCVs produced with Joffset = 0.140 mA cm-2 and tpw = 4 min (i.e. 65.5 ± 2.3), although these PC structure show considerably weaker resonance bands as compared to their counterparts produced at higher JOffset (i.e. > 0.140 mA cm-2). The dependency of QC on Joffset increases within the range 0.140-0.420 mA cm-2, as denoted by the denser color fields with short distance between adjacent field lines.


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Figure 12. Combinational effect of current density offset (JOffset) and pore widening time (tpw) on the resonance band of NAA-µCVs produced by ASPA (note: horizontal dotted black lines denote the baseline (y0) used for the Gaussian fittings, which correspond to the lower lobe of the PSB, and vertical dotted red lines indicate the central wavelength of the resonance band (λR) and the symmetry of the Gaussian fitting). a) JOffset = 0.140 mA cm-2, b) JOffset = 0.280 mA cm-2, c) JOffset = 0.420 mA cm-2, and d) JOffset = 0.560 mA cm-2. e) Contour map showing the dependency of QC with JOffset and tpw for NAA-µCVs produced with JOffset = 0.140–0.560 mA cm-2. f) Contour map showing the dependency of λR with JOffset and tpw for NAA-µCVs produced with JOffset = 0.140– 0.560 mA cm-2.

The broad color fields and more separated field lines at the region of high Joffset and long tpw suggest a weak dependency of QC with these combinations of fabrication parameters, which worsens the quality of the microcavity structure. The average QC estimated for NAA-µCVs as a function of JOffset was 39.1 ± 18.4 and five of these NAA-µCVs showed slightly superior light-confining performance than those reported in previous studies (i.e. QC = 61.7 ± 2.2 at JOffset = 0.140 mA cm-2 and tpw = 0 min, QC = 62.5 ± 3.4 at JOffset = 0.140 mA cm-2 and tpw = 2 min, QC = 65.5 ± 3.6 at JOffset = 0.140 mA cm-2 and tpw = 4 min, QC = 61.9 ± 2.4 at JOffset = 29 ACS Paragon Plus Environment


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0.140 mA cm-2 and tpw = 6 min, and QC = 63.1 ± 1.2 at JOffset = 0.280 mA cm-2 and tpw = 2 min). The effect of Joffset and tpw on the position of the resonance band of NAA-µCVs is summarized in the contour map shown in Figure 12f. This contour map shows that the field line distances at low Joffset (i.e. Joffset < 0.280 mA cm-2) and high Joffset (i.e. Joffset > 0.420 mA cm-2) are relatively wide. However, the color fields become closer with shorter equidistant field lines for Joffset between 0.280 and 0.420 mA cm-2, which indicates a stronger dependency of λR with Joffset within that range of fabrication parameters. It is verified that λR is red-shifted towards the NIR region by increasing Joffset. The higher Joffset is, the longer the wavelength at which light is confined within the structure of NAA-µCVs produced by ASPA. The maximum value of λR (i.e. 1096 ± 1 nm) is located at Joffset = 0.560 mA cm-2 and tpw = 0 min. An increase in tpw has an opposite effect to that of Joffset on the shifting of λR. However, Joffset has a more significant effect on the position of the resonance band as denoted by the color field distribution. CONCLUSIONS In summary, this study has demonstrated that a rational design of the nanoporous structure of NAA-based optical microcavities using apodized stepwise pulse anodization can lead to an enhancement of the light-confining capabilities of these PCs. The structure of these optical microcavities is composed of two apodized NAA-DBRs, which can confine light efficiently (i.e. QC = 112.6 ± 5.2). Furthermore, this nanofabrication approach enables the fine-tuning of the optical properties of the two highly reflective mirrors so light can be confined within the PC structure more efficiently across the spectral regions. The optical properties of NAA-based optical microcavities were assessed in terms of quality factor, position of resonance band, and interferometric colors. The anodization parameters investigated were anodization period, anodization time, current density offset and pore


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widening time. A systematic modification of these parameters allowed to establish optimization paths toward more efficient light-confining NAA-based PC structures. In general, our study established that a combination of longer anodization time, longer anodization period, short pore widening time, and moderate current density offset generates optical microcavities with high quality factor, where the most optimal NAA-µCV was that produced with 20 h anodization time, 1200 s anodization period, 2 min of pore widening, and 0.280 mA cm-2 of current density offset. Our results provide a better understanding and solid foundation to further enhance the light-confining capabilities of NAA-based optical microcavities, opening new opportunities for further fundamental and applied research for these nanoporous PC structures in optical sensing, photonics, and optoelectronics. Supporting Information The Supporting Information file provides information about the structure of NAA-µCVs at different pore widening times, anodization profiles of NAA-µCVs produced by ASPA at different anodization periods, from 800 to 1300 s, and current density offsets, from 0.140 to 0.560 mA cm-2, and a compilation of the values of y0, λR, FWHMR, QC, and R2. Author Information Doctor Abel Santos Research for Impact Fellow – Lecturer School of Chemical Engineering – The University of Adelaide Phone: +61 8 8313 1535 Email: [email protected] Web page: http://www.adelaide.edu.au/directory/abel.santos Professor Andrew Abell Professor of Chemistry and node director of the ARC Centre of Excellence for Nanoscale Biophotonics School of Chemistry and Physics – The University of Adelaide Phone: + 61 8 8313 5652 31 ACS Paragon Plus Environment


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Email: [email protected] Web page: http://researchers.adelaide.edu.au/profile/andrew.abell#contact-details

Acknowledgements Authors thank the support provided by the Australian Research Council (ARC) through the grants number DE140100549 and CE140100003, the School of Chemical Engineering, the University of Adelaide (DVCR initiative ‘Research for Impact’), the Institute for Photonics and Advanced Sensing (IPAS), and the ARC Centre of Excellence for Nanoscale BioPhotonics (CNBP).

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